Grain refinement of metallic components by controlled strain path change

ABSTRACT

Nowadays, the production of metallic parts, namely sheet metal, with high strength and formability, represents a big challenge for automotive, aerospace and assembling industries. The present invention deals with a new plastic deformation process that allows, by controlling the strain path change, the production of sheet metals with a grain size of 1-2 micron and, consequently, with a yield stress 3-4 times than the ones obtained with conventional processes, keeping the same formability. The process is easy for industrialization and its optimization requires only two fundamental parameters that characterize the ratio of thickness reduction, shear strain and amplitude in the change of strain path.

TECHNICAL DOMAIN OF THE INVENTION

Nowadays, the production of metallic parts, namely sheet metal, with high strength and formability represents a big challenge for automotive, aerospace and assembling industries. Weight reduction, saving of raw materials and less pollutants emission are the major driving forces for the utilization of lightweight materials in automotive and assembling industries.

Rolling is the conventional technology for the production of metal sheets. In this process the final sheet thickness is dictated by the distance between two rolls (upper and lower) that rotate with the same speed. After rolling, the material generally presents a rolling texture and re-crystallization treatments are necessary to carry out in order to recover the initial properties of the material. After the heat treatment the materials present a grain size usually ranging between 20 and 65 micron. It must be emphasized that in agreement with the Hall-Petch law, the yield stress is inversely proportional to the grain size.

New metals and alloys have been developed to satisfy the new requirements of production, namely high strength steels and aluminum alloys of the series 5XXX, 6XXX and 7XXX. Alloying the basic materials allowed higher strengths to be obtained. However, new technological problems have appeared with these new alloys. For instance the high strength steels present an important spring-back phenomenon which is necessary to compensate during plastic deformation. The aluminum alloys also present a negative strain rate sensitivity which represents a big limitation for mass scale production.

In this context, the development of a technology that allows the production of sheet metals with refined grains and with a crystallographic texture compatible with high r value and consequently high formability is of extreme importance for the needs of the above-mentioned industries.

SUMMARY OF THE INVENTION

The present invention deals with a process of grain refinement of metals by means of a compression-shearing strain state associated with a change in strain path. The change in strain path is obtained by alternating the relative speeds of the upper and lower rolls after each rolling step.

In a first preferential approach, the process of grain refinement of metals by means of a compression-shearing strain state associated with a change in strain path, obtained by alternating the relative speeds of the upper and lower rolls after each rolling step, is applied to sheet metals.

In a second more preferential approach, the process deals with plastic deformation of materials under successive compression-simple shear, at least in two successive rolling stations by alternating the relative speeds of the upper and lower rolls.

In another approach, also preferential, the amplitude in the change of strain path (α) is lower than 0,5. In a more preferential approach the amplitude in the change in strain paths (α) is located in the range −0,13 -0,5. In other approach more preferentially, the amplitude in the change in strain path is located in the range −0,13 -0,13.

In another approach, also preferential, the parameter α is higher than 1, as expressed by the relationship between the thickness reduction ratio r and the apparent shear strain, γ_(app):

$\gamma_{app} = {a\frac{r\left( {2 - r} \right)}{2\left( {1 - r} \right)^{2}}}$

In a more preferential approach, the parameter α is in the range 1.2 to 2.5. In an approach much more preferential, the parameter α is in the range 1.75-2.5.

In another approach of this invention, also preferential, the new process allows the production of sheet metals with ratios of thickness reduction higher than 500%. In other preferential approach, the process allows the production of sheet metals with a grain size of 1-2 micron that can be further stabilized by heat treatments at moderate temperatures. In other approach, also preferential, the process allows the production of sheet metals with a yield stress 3-4 times higher with respect to the conventional rolling process, keeping the same level of formability.

In a preferential approach of this invention, the process takes place in one or more working stations were the material is plastically deformed under compression-simple shear by alternating the relative speeds of the upper and lower roll in each working station.

In another approach of this invention, the metals produced by this technology, present refined grains as the result of the simultaneous effects of compression-simple shear and change in strain path obtained by alternating the relative speeds of the upper and lower roll in each working station.

In a preferential approach of this invention, the reported metallic component is a metal sheet. In a more preferential approach, this metal is a steel or an aluminum alloy.

Previous Developments of the Process

In the past few years, the rolling process has suffered new developments. For instance, in the document U.S. Pat. No. 5,666,842, special emphasis was given to the magnetic properties of metals along the rolling direction, due to the alignment of grains and subsequent primary and secondary re-crystallization. In another document, the U.S. Pat. No. 3,861,188, special care was given to the influence of the thickness reduction per pass. In this case, the sheet flows between the rolls in a serpentine like shape and is subjected to tension and back tension. This process has a big economic impact. However, it does not give the solution for obtaining, simultaneously, high strength and formability in the rolled sheets. On the contrary, in the present invention, the reduction per pass is not the fundamental parameter. In fact, the present process imposes a simple shear strain state along the entire sheet thickness. This means that the reduction pass must be analyzed with respect to the shear strain after each step of the process. Moreover, the optimized control of the strain path between the working stations is of crucial importance in the present invention.

In this way, it is possible to conciliate high strength by grain refinement and high formability by producing an optimized anisotropy under dynamic recovering processes.

FIGURES DESCRIPTION

FIG. 1: Schematic representation of the transformation of a rectangular shape element defined by OABC into the parallelogram shape element defined by OA′B′C′ following a plane strain state without volume change. (1) represents the OABC section and (2) the OA′B′C′ section.

FIG. 2: Evolution of the apparent shear strain as a function of the thickness reduction ratio for different α values.

FIG. 3: Schematic representation of the change in strain path that occurs between two successive deformation steps. (3) represents the sheet before the strain path change and (4) the sheet after strain path change.

FIG. 4: Schematic representation of the plastic deformation process along the different steps. It can be noticed that in each working station the relative speeds of the upper and lower rolls are successively inverted. (5) is the sheet metal, (6), (7), (8) and (9) represent the upper rolls and (10), (11), (12) e (13) represent the lower rolls: the speed of the upper rolls (6, 8) is higher than the speed of the lower rolls (10, 12), and the speeds of the upper rolls (7, 9) is lower than the speeds of the lower rolls (11, 13).

FIG. 5: Schematic representation of the shear deformation along the sheet thickness. It can be seen, that the shear strain can be measured directly from angle θ. (14) represents the sheet metal before deformation and (15) the sheet metal after deformation.

FIG. 6: Schematic representation of the evolution of α with a.

FIG. 7: Schematic representation of the mechanical properties of Aluminum (AA1050-O) of series 1XXX obtained in uniaxial tension after the process. For comparison the mechanical properties of the same material after conventional rolling and re-crystallization is shown.

INVENTION DESCRIPTION

The present invention deals with a new rolling process that allows, simultaneously, the increase of strength and formability of sheet metals. For this purpose, the sheet metal (1, 2) is deformed (FIG. 1) in successive steps (FIG. 4) with the following specifications: In each plastic deformation operation, i.e., working station, the speeds of the rolls (6, 10) are different in order to produce a shear deformation along the entire sheet thickness. The shear strain can be measured directly from the sheet (14, 15) through the angle θ as shown in FIG. 5. The relationship between the ratio of thickness reduction r with the apparent shear strain γ_(app), that in turn is related with angle θ by γ_(app)=tan(θ), is established by the equation

$\gamma_{app} = {a\frac{r\left( {2 - r} \right)}{2\left( {1 - r} \right)^{2}}}$

The evolution of the apparent shear strain with the ratio of thickness reduction for different a values is illustrated in FIG. 2.

In the second working station (7, 11) the relative speeds between the upper and lower rolls are inverted (FIG. 4). Under optimized conditions, a change in strain path will occur (FIG. 3), while the thickness of material is reducing and the shear strain increasing. The change in strain path is characterized by the parameter a expressed by the equation:

$\alpha = \frac{1 - \frac{a^{2}}{4}}{1 + \frac{a^{2}}{4}}$

The evolution of a with α is shown in FIG. 6. The amplitude in the change of strain path (α) is defined by the cosines of the angle defined by the two vectors which represent the strain rate of the successive deformations.

As can be understood, one of the key requirements of the process consists in obtaining an optimized strain path change. In this process, the change in strain path is induced by alternating the relative speeds of the upper and lower rolls in each working station. In a first step the metal sheets are subjected to a compression between rolls being the speed of the upper roll (6) higher than the speed of the lower roll (10) and, in a second step, to a compression were the speed of the lower roll (11) is higher than the speed of the upper roll (7). This sequence can be repeated (8, 9, 12, 13) until the final thickness is obtained.

The intense shear strain along the entire thickness of the sheet produces grain refinement and high anisotropy, compatible with the high strength and formability requirements of plastic deformation processes. The change in strain path is responsible for the processes of dynamic recovery and, consequently, contributes positively for the material formability.

The materials should be subject to stabilization heat treatments for 10-20 min at low temperatures.

The principal technological parameters should be extracted from the graphic α vs a in FIG. 6.

The optimized working conditions are: α between 0.13 and −0.13 and α between 1.75 and 2.5.

In general, the conditions of the process for which the improvement of formability is noticed are: α between 0.5 and −0.13, and α between 1.2 and 2.5.

It was experimentally demonstrated that for α values <0.5 and a values >1 the results are already positive, and improvements in strength and formability with respect to conventional rolling are remarkable.

In general terms, after the process, metals and alloys present a grain size of 1-2 micron and can be easily stabilized by a thermal treatment at low temperatures. The materials present a yield stress 3-4 times higher than the one obtained by conventional rolling, keeping the same degree of formability. This process allows high ratios of thickness reduction (>500%).

As an example, we show in FIG. 7 the mechanical properties of aluminum alloy AA1050-O of series 1XXX obtained in uniaxial tension. For comparison we show in the same figure the mechanical properties of the same material after conventional rolling and re-crystallization.

This technology can easily be adapted for the production of different metallic profiles. 

1. Process of grain refinement by controlled strain path change under compression simple shear plastic deformation during successive steps were the relative speeds of the upper and lower rolls are alterated.
 2. Process of grain refinement by controlled strain path change under compression simple shear plastic deformation during successive steps were the relative speeds of the upper and lower rolls are alterated, according to claim 1, with special emphasizes in sheet metal.
 3. Process of grain refinement by controlled strain path change under compression simple shear plastic deformation, according to claim 2, occurring the process in at least 2 working stations where the relative speeds of the upper and lower rolls are alterated.
 4. Process of grain refinement by controlled strain path change, according to claim 1, wherein an amplitude of the change in strain path (α) is lower than 0.5.
 5. Process of grain refinement by controlled strain path change, according to claim 4, wherein an amplitude in the change in strain path (α) is located in the range −0.13-0.5.
 6. Process of grain refinement by controlled strain path change, according to claim 5, wherein an amplitude in the change in strain path (α) is located in the range −0.13-0.13.
 7. Process of grain refinement by controlled strain path change, according to claim 1, wherein, a parameter “a” higher than 1 is expressed by the following relation between the thickness reduction ratio r and the apparent shear strain γ_(app): $\gamma_{app} = {a{\frac{r\left( {2 - r} \right)}{2\left( {1 - r} \right)^{2}}.}}$
 8. Process of grain refinement by controlled strain path change, according to claim 7, wherein a parameter α is in the range 1.2-2.5, as expressed by the following relation between the thickness reduction ratio r and the apparent shear strain γ_(app): $\gamma_{app} = {a{\frac{r\left( {2 - r} \right)}{2\left( {1 - r} \right)^{2}}.}}$
 9. Process of grain refinement by controlled strain path change, according to claim 8, wherein a parameter “a” is in the range 1.75-2.5 as expressed by the following relation between the thickness reduction ratio r and the apparent shear strain γ_(app): $\gamma_{app} = {a{\frac{r\left( {2 - r} \right)}{2\left( {1 - r} \right)^{2}}.}}$
 10. Process according to claim 2 further comprising production of metal sheets with thickness reduction ratios higher than 500%.
 11. Process according to claim 2 further comprising production of sheet metals with grain sizes around 1-2 micron that can be stabilized by thermal treatments at low temperatures.
 12. Process according to claim 2 further comprising production of sheet metals with a yield stress 3-4 higher than the one obtained in conventional processes without alternating the relative speeds of the rolls, keeping the same degree of formability.
 13. Equipment for grain refinement of metallic parts by controlled strain path change according to claim 1, including one or more working stations where the material is plastically deformed under compression-simple shear and where the relative speeds between the upper and lower rolls are alternated after each pass.
 14. Metallic part produced according to claim 1, including a grain refinement, obtained as a result of controlled strain path, plastic deformation by compression simple shear in successive steps and alternating the relative speeds of the upper and lower rolls after each deformation step.
 15. According to claim 14, wherein the metallic part is a metal sheet.
 16. According to claim 15, wherein the metal sheet is steel or aluminum sheet. 